Conventional power conversion devices control a current relating to driving of a rotary electric machine through pulse width modulation (hereinafter, referred to as PWM) as disclosed in Japanese patent No. 3687861 (U.S. Pat. No. 5,933,344) and Japanese patent No. 2577738 (U.S. Pat. No. 4,847,743). In a case where the rotary electric machine is a three-phase motor, to improve the voltage use rate in PWM control, two-phase modulation is performed, in which a voltage of one phase is fixed and only the other two-phases are modulated as disclosed in “Semiconductor Power Conversion Circuit”, OHM Ltd., 1987, Institute of Electrical Engineers of Japan.
As methods of controlling PWM, a triangular wave comparing method and an instantaneous voltage vector selecting method are known. For example, in the triangular wave comparing method, when any one phase of a command signal (hereinafter, referred to as a duty command value) relating to a voltage applied to each phase of a coil of a rotary electric machine is constantly 100% or 0%, the above-described two-phase modulation is performed, whereby the highest inter-line voltage can be output. In addition, the closer to 0% or 100% the duty is, the higher the inter-line voltage that is applied.
In addition, in the conventional power conversion device, to prevent formation of a short circuit due to simultaneous turning on of a high electric potential-side switching device and a low electric potential-side switching device of each phase, a dead time is provided so that the high electric potential-side switching device and the low electric potential-side switching device are turned off. In a case where the dead time is provided, a duty command value close to 0% or 100% cannot be output depending on the microcomputer, and an applicable inter-line voltage is limited. Thus, in Japanese patent No. 3687861 (U.S. Pat. No. 5,933,344), duty that is close to 0% or 100% can be output by changing the configuration of a counter circuit arranged inside the microcomputer. However, according to Japanese patent No. 3687861 (U.S. Pat. No. 5,933,344), a special circuit is necessary, and accordingly, the configuration is complicated.
In addition, in a case where the dead time is provided, the inter-line voltage is distorted due to the influence of the dead time, whereby a current is distorted. Accordingly, a torque ripple is generated, or vibration or noise is generated. Thus, according to Japanese patent No. 2577738 (U.S. Pat. No. 4,847,743), three-phase modulation is performed at the time of a low voltage, and two-phase modulation is performed at the time of a high voltage, whereby the distortion of the voltage due to the influence of the dead time is decreased. However, in Japanese patent No. 2577738 (U.S. Pat. No. 4,847,743), the influence of the dead time cannot be avoided at the time of a high voltage.